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Pitfalls to adding cellulose in unvented attic?

I'm helping a homeowner improve his 1974 ranch, zone 5 Ohio. Attic currently has 6" of cellulose, no air sealing. Blower door results were predictably awful. After removing all recessed light fixtures and fully air sealing the attic floor, I'll be adding more cellulose. There are no mechanicals in the attic, except for a plumbing vent and a little wiring.

Question: are there any potential problems in simply blowing in more cellulose? Even up to touching the roof sheathing near the eaves? (It wouldn't take much, in fact, to completely fill the attic, if one was inclined..)

Issue: there is essentially no venting of the attic space. I understand that this is not necessarily a problem--there is no evidence of moisture damage anywhere on the roof deck or the ceiling below, so neither vapor nor ice damming seem to have been an issue in the roof's first 40 years.

The roof is low-pitch (only 4:12) and constructed with 2x6 rafters, which extend past the wall for a nice 4' overhang. There is no gable or soffit venting, and fiberglass batts are stuffed in the 5.5" space above the wall plate to keep loose fill out of the overhang. (With no venting, there is no wind washing.) Ridge venting was cut in with the most recent asphalt roof, just that nominal stuff that looks as ineffective as it is. My worry is that perhaps the space HAS been vented, via the ceiling air leaks and the ridge venting, and that air sealing and extra fill will change the thermodynamics completely.

Any analysis and wisdom would be appreciated!

Asked by Andy Chappell-Dick
Posted Aug 20, 2014 9:17 AM ET
Edited Aug 20, 2014 9:50 AM ET


7 Answers

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The right way to prepare this attic for more cellulose would be to install soffit vents in the existing soffit, and ventilation baffles (Proper-Vents or AccuVent baffles) in each rafter bay to connect the soffit with the area above the top of the cellulose insulation.

Ideally, you would also install new airtight insulation dams between the rafters, near the eaves (above the top plate of the perimeter walls).

If these details are new to you, you might want to watch this GBA video: How to Ventilate Rafter Bays When Adding Insulation.

Answered by Martin Holladay
Posted Aug 20, 2014 9:52 AM ET
Edited Aug 20, 2014 9:54 AM ET.


Since your new air sealing theoretically keeps the attic free of internally generated moisture, that leaves externally sourced humidity and condensation in your cellulose to dry to the outside. Cell is very capable of handling routine moisture cycles like that. Borate treated cellulose will not support fungi that might start decay. For this reason I am sympathetic to what you're considering.

If your loose fill cell layer is deep enough to keep the dew point outside of your air sealing layer (across most of the attic), why be shy about adding good air flow using gable vents and working in tandem with the new ridge vent. I think that bulk air vents (gable end louvers for example) are more important than adding vents from each soffit to attic space. (This is not too popular a kind of thinking here but some cell manufacturers agree and code officers will sometimes allow a variation if the recommendation comes from the industry). That will increase the attic handling of routine needs for drying and evaporation. I might personally consider this instead of Martin's code compliant and the CIMA recommended vents between the soffits and the attic.

There are a couple of options recommended by CIMA:




Since the plate to roof space adjoining the soffits are your weak point (presuming your air sealing details are done well there), will the cellulose in the narrow space between the roof and plate be a place for uncontrollable and too risky a quantity of condensation to accumulate?

How did you air seal the top plate's and wall sheathing top seams to keep conditioned air from getting into the unvented soffit side of the roof?

Remember, too, that GBA-FHB is liable for the advice of its online experts.

Answered by flitch plate
Posted Aug 20, 2014 11:29 AM ET
Edited Aug 20, 2014 11:33 AM ET.


Martin, doing it the "right" way is problematic. Access to the top of the wall (attic eaves) is virtually impossible without expensive demolition, and the vertical space is 5.5" at the edge. So I can't reach to perform proper airsealing or install proper dams, and even if I could the low clearance means there's not enough insulation--with or without air channel--to confidently prevent condensation. Flitch argues for bulk gable venting; but I only have one gable--the other end is closed up against a perpendicularly-gabled cathedral ceiling.

One of the "controversial" methods described in your 2013 blog, the second link provided by Mr. Plate, is dense-packing the inaccessible areas of the attic (not more than 1/3 of total area) as an alternative to impossible-to-reach airsealing and costly demolition. Done right, this seems like a good low-cost strategy in my zone, but I'm not comfortable with the risk level and lukewarm endorsement.

Best approach will have to be to tear off soffit in order to access top of wall. [Maybe the BEST approach of all, in the spirit of Flitch's last comment, would be to do nothing. What would you other contractors do in this scenario? It's not my own house.]

Next problem: the walls have no sheathing and thus no air barrier. 2x4 studs covered with fuzzboard, gapped loosely every 24" with horizontal 1x3 firring strips for attaching vertical siding. Steel bracing present for racking, might have plywood at the corners. I figured concentrating on a leaky ceiling, along with conditioning the crawlspace, was a better place to spend their efficiency dollars at first.

Answered by Andy Chappell-Dick
Posted Aug 20, 2014 1:25 PM ET


Piling in the cellulose without addressing the venting deficiencies is certainly an option. Plenty of insulation contractors do that. As long as you (and the homeowner) are doing it with open eyes, and with awareness of the risks (as well as the fact that the method is not usually code-approved), it's not unreasonable.

By the way, removing a soffit to gain access to the tricky area we are discussing isn't usually that difficult. It's probably the easiest way to go if you decide to install ventilation chutes.

It sounds like the walls are leaky. The best way to address it would probably be to remove the existing siding and to install a continuous layer of rigid foam (with taped seams) on the exterior. Then install new furring strips and new siding. Since this work is very expensive, and would have a very long payback if done for energy reasons alone, it's reasonable not to do it.

Another option: install dense-packed cellulose in all the stud bays. Dense-packed cellulose cuts down on air leakage.

Answered by Martin Holladay
Posted Aug 20, 2014 1:47 PM ET


Dense packed cellulose would be great if it can be done without creating other problems, but it might no be possible in the described assembly:

" 2x4 studs covered with fuzzboard, gapped loosely every 24" with horizontal 1x3 firring strips for attaching vertical siding. "

It's the "...gapped-loosely..." clause that inserts the question mark. If it's loose enough that you end up with cellulose wadded between the fuzz-board and siding it can wick copious quantities of water from the siding into the wall cavity, whereas the furring would other wise be establishing an air channel, which is a very good capillary break. If the fiberboard didn't have big enough gaps that not much cellulose ended up in the channel it would tighten up the assembly considerably. It all boils down to just what "...gapped-loosely..." really means. An 1/8" gap between sheets isn't a big deal- the cellulose would clog & seal it, but 1/4" gap could potentally be a problem. Half-inch gaps would be downright asking for trouble.

Answered by Dana Dorsett
Posted Aug 20, 2014 2:27 PM ET


Thanks for all the feedback. Let me zero in on a question that all this raises for me--that of how contractors handle the risk factor of different strategies, and how they work with clients in determining the best path. I have been a general remodeling contractor, but am entering the field of energy retrofits. Energy-conscious clients tend to be somewhat motivated to engage the issues, more so than the general remodeling clients, but in my experience so far folks tend to rely almost entirely on my explanations and suggestions. I urge them to seek a second opinion, but in my area there are virtually no other contractors offering ANY advanced energy solutions. [And to add to backwater feel, we are under the jurisdiction of NO building code enforcement for new construction, let alone for remodeling. You would not believe what is still getting built around here.]

How do you contractors end up deciding on the best path forward, when there are multiple options and the client is relying heavily on your judgment?

How do you contractors handle the question of risk with your clients? I assume you sign an agreement protecting you (the contractor) from moisture-related lawsuits down the road?

Answered by Andy Chappell-Dick
Posted Aug 20, 2014 5:10 PM ET


I would run the other way if you asked me to sign away your liability (you the contractor, me the home owner). I am more for your current approach. Consult, consult and consult. Review options and make a recommendation. Have them sign off on the plan, not the liability. If you agree with their decisions, you might want a caveat in the agreement that you recommend something else, but that sparks of a poor relationship to begin with ... why take on that kind of headache. If its bad to start, it will be worse to end.

Answered by flitch plate
Posted Aug 21, 2014 10:40 PM ET

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